That's a 3.4GHZ P4 there hox,
Anyways, I'll try and help you out as best I can.
I have never tried dual booting Linux & OS X, so I don't know for sure, but I don't see why not. There is also the GIMP for Mac: http://www.macgimp.org/
Which I see that you found too when I saw the edit line in your post.
When I tried out a Mac mini at the Apple Store about a month ago, I noticed they were pretty solid systems, and the speed was good. Nothing like the G5 of course, but for what he would be doing on it, it should work fantastic.
For CD & DVD Burning, I use Roxio Toast, it works great for what I do with it, burning images, data cd's, etc.
External device support is very good. I'll be honest, I never figured out how to get a printer hooked up on a PC to print over the network from the Mac, but hooked up directly to the computer, it's a piece of cake. For scanners, I believe both Epson & Canon models work with it.
It is basically a no upgrade system, you might be able to upgrade RAM, but that would be it, and I'd recomend starting off the machine with 512MB RAM, 256 just isn't enough.
For software, I'd recomend checking out: http://www.versiontracker.com
Linux/GIMP to Panther/Photoshop performance, I wouldn't know.
As far as running KDE on OS X, good luck, I highly doubght it can be done.
As you already mentioned, the Ease of Use & Security of the Mac makes it easier for everyone, which is why I highly recommend it.
Feel free to ask any other questions.
I'd also recommend the forums over at http://www.macrumors.com